Self-propelled pneumatic tools are used to form holes for pipes or cables beneath roadways without need for digging a trench across the roadway. These tools include, as general components, a torpedo-shaped body having a tapered nose and an open rear end, an air supply hose which enters the rear of the tool and connects it to an air compressor, a piston or striker disposed for reciprocal movement within the tool, and an air distributing mechanism for causing the striker to move rapidly back and forth. The striker impacts against the front wall (anvil) of the interior of the tool body, causing the tool to move violently forward into the soil. The friction between the outside of the tool body and the surrounding soil tends to hold the tool in place as the striker moves back for another blow, resulting in incremental forward movement through the soil. Exhaust passages are provided in the tail assembly of the tool to allow spent compressed air to escape into the atmosphere.
Most impact boring tools of this type have a valveless air distributing mechanism which utilizes a stepped air inlet. The step of the air inlet is in sliding, sealing contact with a tubular cavity in the rear of the striker. The striker has radial passages through the tubular wall surrounding this cavity, and an outer bearing surface of enlarged diameter at the rear end of the striker. This bearing surface engages the inner surface of the tool body.
Air fed into the tool enters the cavity in the striker through the air inlet, creating a constant pressure which urges the striker forward. When the striker has moved forward sufficiently far so that the radial passages clear the front end of the step, compressed air enters the space between the striker and the body ahead of the bearing surface at the rear of the striker. Since the cross-sectional area of the front of the striker is greater than the cross-sectional area of its rear cavity, the net force exerted by the compressed air now urges the striker backwards instead of forwards. This generally happens just after the striker has imparted a blow to the anvil at the front of the tool.
As the striker moves rearward, the radial holes pass back over the step and isolate the front chamber of the tool from the compressed air supply. The momentum of the striker carries it rearward until the radial holes clear the rear end of the step. At this time the pressure in the front chamber is relieved because the air therein rushes out through the radial holes and passes through exhaust passages at the rear of the tool into the atmosphere. The pressure in the rear cavity of the striker, which defines a constant pressure chamber together with the stepped air inlet, then causes the striker to move forwardly again, and the cycle is repeated.
In some prior tools, the air inlet includes a separate air inlet pipe, which is secured to the body by a radial flange having exhaust holes therethrough, and a stepped bushing connected to the air inlet pipe by a flexible hose. These tools have been made reversible by providing a threaded connection between the air inlet sleeve and the surrounding structure which holds the air inlet concentric with the tool body. The threaded connection allows the operator to rotate the air supply hose and thereby displace the stepped air inlet rearward relative to the striker. Since the stroke of the striker is determined by the position of the step, i.e., the positions at which the radial holes are uncovered, rearward displacement of the stepped air inlet causes the striker to hit against the tail nut at the rear of the tool instead of the front anvil, driving the tool rearward out of the hole.
Wentworth et al. U.S. Pat. No. 5,025,868 describes a ground-piercing tool having an improved form of screw-reverse mechanism, a striker having annular bearing rings at each end, and a removable, axially clamp-loaded end-cap assembly that facilitates repair and reassembly of the tool. Wentworth et al. U.S. Pat. No. 5,199,151 describes a tool of similar construction wherein the tool body is made by rotary swaging rather than by machining a solid metal bar.
A common disadvantage of the known screw reverse mechanism is the need to rotate the hose through several revolutions in order switch from forward to reverse mode. This must be done manually and can be difficult when the tool has travelled a long distance because of the length of hose that must be twisted. As a result, several improved forms of ground piercing tools have been developed that provide mechanisms for a quarter- or half-turn to switch from forward to reverse mode. See generally Bouplon U.S. Pat. No. 4,662,457, Jenne U.S. Pat. No. 5,307,883, and Kayes U.S. Pat. No. 4,618,007. Kayes U.S. Pat. No. 5,318,135 in particular provides a reversing mechanism that relies on an air supply tube having a side opening that can be selectively rotated into alignment with the radial port normally provided in the striker in order to introduce compressed air prematurely into the forward chamber of the tool in order to shorten the forward stroke of the striker for reverse travel. A resilient detent mechanism is provided as part of the tail assembly for locking the inner tube into its operative positions. These devices have proven useful in practice but are generally more complex and hence more expensive to manufacture than the basic screw reverse mechanism.